Technical Field
The invention concerns a method for measuring the thickness on measurement objects in general, whereby at least one sensor measures against the object from the top and at least one other sensor measures against the object from the bottom. With a known distance between the sensors, the thickness of the object can be calculated in accordance with the generally known formula D=Gap−(S1+S2), whereby D=thickness of the measurement object, Gap=distance between the sensors, S1=distance of the top sensor to the upper side of the measurement object and S2=the distance of the top sensor to the underside of the measurement object. The invention further concerns a device for applying the method.
Related Art
In the industrial measuring field, the thickness of measurement objects is usually measured without contact by means of distance sensors, by measuring against the upper side of the measurement object with one sensor. Another sensor measures against the underside of the measurement object. With a known spacing of the sensors to one another, the thickness can be calculated according to the aforementioned formula. However, this mathematical relationship is true only if the sensors relative to one another relative to the measurement object and are aligned in an optimal manner, as shown in FIG. 1 in a schematic view. In practice, there are two major sources of error, namely the tilting of the measurement object, and/or the displacement and the possible tilting of the sensors.
As soon as the measurement object tilts—even when the sensors are in ideal alignment to one another—a thickness is measured that is greater than the actual thickness of the measurement object. This is due to the angle error. FIG. 2 shows the related measurement error, which occurs when the measurement object is tilted in the angular range from −30° to +30°.
The second source of error is the orientation of the sensors to one another, namely, whether they are aligned to one another and/or are tilted in relation to one another. If the sensors are misaligned and their measurement axes do not lay 100% on top of one another, the tilting of the measurement object, or its displacement within the measuring gap between the sensors, results in additional variances in the calculation of the thickness. FIG. 3 shows a thickness measurement using two laser distance sensors that are oppositely disposed, offset from one another. The measurement object is also tilted, with offset sensor axes. A tilting of the sensors can occur as well, namely, an angle error in the alignment of the sensors, which leads to further measurement errors.
Ideally, the two sensors lie on one axis, so that a tilting of the measurement object always results in a larger thickness value. However, due to mechanical tolerances, or due to the fact that the laser spot in laser sensors scatters quite considerably on the measurement object, which makes the alignment of the laser sensors more difficult, this cannot be achieved in practice. In addition, the laser beam does not correspond exactly to the ideal linearity axis of the sensor. In practice, a tilting of the measurement object can lead to a smaller thickness value, because the error is dependent on the error in the laser adjustment, as well as, in absolute terms, on the thickness of the measurement object.
FIG. 4 shows the development of the measurement error, which is caused only by an incorrect adjustment of the sensors (distance sensors).
The above-identified errors are measurement errors for distance sensors that perform a point measurement. The thickness measurement can similarly be conducted with sensors, which project a line to measure (for example laser line scanners, light section sensors), or allow a two-dimensional measurement (for example matrix arrays or cameras). Even when using laser line scanners, the measurement will be incorrect if tilting of the measurement object occurs with a simultaneous displacement or tilting of the sensors.
By using line scanners or planar sensors, the tilt angle of the measurement object can be determined in addition to the distance. With the help of the additional information concerning the tilt angle of the measurement object, it is possible to correct the previously identified errors, so as to be able to offset the thickness error caused by the tilt.
The aforementioned method for measuring thickness is typically used in systems with C-frames or O-frames. In the C-frame, the two distance sensors are mechanically fixed, or allocated, to one another. For traversing measurement of objects with a larger width, the entire C-frame is moved over the measurement object (or vice versa) and the thickness profile of the measurement object is recorded. The initial adjustment error does not change across the traversing width, i.e. the error is constant and independent of the x-direction.
The two distance sensors can also be installed in an O-frame. The sensors are respectively mounted on a shaft and are moved by a motor, for example, via a toothed belt. For mechanical reasons, the previously discussed laser adjustment error, which is dependent on the C-frame as well, also changes as a function of the position of the sensors in traversing direction.
The alignment of the sensors would not cause an additional measurement error, if one could ensure that the measurement object is always in the same position in relation to the sensors. However, since in a real-world production environment there are always variances in positioning, tilting of the measurement object in relation to the sensors is commonplace. The determination of such tilting by means of line sensors, by means of which a calibration is performed, is known per se from practice.
The known solution is disadvantageous, however, insofar as the sensors used therein have to be precisely aligned with one another. Once a displacement of the sensors occurs, the measurement error can no longer be corrected.
A precise alignment of the sensors is more difficult, the larger the production plant. Due to unavoidable, not insignificant mechanical tolerances, the measurement axes of the sensors cannot be precisely aligned to one another. The use of special means of adjustment, for example micrometer screws, etc., is expensive, and their application is complicated. In addition, this type of fine adjustment is difficult to accomplish in harsh industrial environments.